1. Field of the Invention
This invention relates to the manufacture of a magnetic recording medium, and more particularly to a method of smoothing a wet coating on the medium while also dissipating electrostatic charge from the medium.
2. Description of Related Art
The manufacture of magnetic recording media, such as 1/2 inch VHS video tape, typically involves coating a moving web with a solvent-based material. During coating, the web contacts various processing equipment, such as rollers, at high speeds. Since the web is usually an insulating material such as polyester film, this contact can result in the buildup of electrostatic charge (i.e., "static electricity") on the web. The accumulated charge, if not dissipated, can result in a variety of undesirable consequences, including the attraction of foreign substances such as dirt to the surface of the medium. A medium contaminated in this way may in turn be affected by dropouts and head-to-tape spacing loss, thereby depreciating the quality of data transfer from the medium. In addition, if the charge accumulates beyond the breakdown potential of the surrounding air, a spark will be discharged which may ignite the solvent-rich atmosphere in the coating area.
To reduce the occurrence of such discharges and the associated risk of fire, electrostatic charge has been at least partially removed from the moving web using devices such as inductive needle bars, electrically powered bars and nuclear ionizing bars. As the coating speeds of magnetic media continue to increase for efficiency reasons, higher levels of electrostatic charge are expected to be generated, and less time will be available for charge removal by these known devices.
There is a growing interest in the use of antistatic agents to solve the problems created by the buildup of electrostatic charge. Antistatic agents, or "antistats", are electrically conductive materials that are capable of transporting charges away from areas where charge buildup is not desired. For example, antistats such as carbon black may be included in a magnetic or nonmagnetic dispersion coated on the magnetic recording medium itself to reduce the buildup of debris on the medium during converting and packaging. Such efforts to reduce electrostatic charge in the medium are primarily effective only after the coated dispersion is dry.
A process for coating dispersions on magnetic recording media using a method such as rotogravure often includes a smoothing device such as a blade or bar in contact with the wet coating to remove patterns and defects from the wet coating before it is dried, as described for example in U.S. Pat. No. 4,870,920.